1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, and more particularly, to an image forming apparatus including a transfer device that transfers a toner image formed on an image bearing member to a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies a single-component or a two-component developer to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Known image forming apparatuses employ an intermediate transfer method in which toner images formed on one or a plurality of photosensitive members serving as a first image bearing member are transferred onto an intermediate transfer member serving as a second image bearing member so that the toner images are superimposed one atop the other, forming a composite toner image in a process known as primary transfer. Subsequently, the composite toner image on the intermediate transfer member is transferred onto a recording medium such as a sheet of paper, in a process known as secondary transfer.
A transfer device employed in such image forming apparatuses that transfers the toner image from the image bearing member such as the photosensitive member and the intermediate transfer member onto the recording medium often employs a roller-type transfer member that contacts the image bearing member, thereby forming a transfer nip therewith through which the recording medium passes and the toner image on the image bearing member is transferred thereon as the recording medium passes.
It is known that in the image forming apparatuses equipped with such a transfer device, toner tends to be supplied excessively to the image bearing member in a low-temperature, low-humidity environment at the start-up of the image forming apparatus. As a result, the toner sticks undesirably to a non-image formation area or a background portion of the image bearing member. When this occurs, the toner adhering to the non-image formation area or the background portion of the image bearing member migrates to the transfer member, resulting in contamination of the transfer member. The non-image formation area herein refers to an area other than an area where an image is formed.
In order to prevent contamination of the transfer member and the recording medium by toner, JP-2007-133191-A proposes use of a parting agent that enhances separation of toner from the surface of the transfer member. The parting agent is supplied to the surface of the transfer member by a parting agent applicator.
Although effective, the drawback of this approach is that the parting agent is consumable, and a structure to continuously supply the parting agent is required, which is generally expensive.
With reference to FIG. 13, a description is provided of contamination of the transfer member in known image forming apparatuses. In a case in which a user uses mostly recording media sheets of the same size, if the width of a secondary transfer roller 500 serving as a transfer member is not the same as the width of a recording medium P as illustrated in FIG. 13, end portions of the secondary transfer roller 500 in the axial direction thereof which are beyond the width of the recording medium are contaminated by toner. More specifically, the end portions of the secondary transfer roller 500 are contaminated because the toner once adhered undesirably onto an intermediate transfer belt 100 migrates to the end portions of the secondary transfer roller 500 where no recording medium passes, that is, the non-image formation area during the secondary transfer. Such toner accumulates at the end portions of the secondary transfer roller 500 over time.
In order to prevent contamination of the transfer member, various cleaning methods have been proposed.
For example, according to JP-2007-334011-A, a cleaning device is attached to a secondary transfer roller serving as the transfer device. However, although advantageous and generally effective for its intended purpose, there is a drawback to this configuration in that the dedicated cleaning device for the secondary transfer device increases the size and the cost of the image forming apparatus as a whole.
In JP-2008-090015-A, the image bearing member and the secondary transfer roller are separated from one another by a certain distance similar to a recording medium, thereby preventing contamination of the secondary transfer roller. In the meantime, an electric field opposite in charge to the electric field applied at transfer of toner to the recording medium is applied to the secondary transfer roller, thereby returning the toner once adhered to the secondary transfer roller to the image bearing member.
Although effective, the drawback of this approach is that separating the image bearing member from the secondary transfer roller and application of the opposite electric field to the transfer member degrade productivity.
In JP-2003-248361-A, in order to prevent contamination of the transfer member caused by toner in test patterns for adjustment of a toner density, the test patterns are supplied with opposite reversed electric charge by a charging device so that toner in the test patterns once transferred onto the second image bearing member are returned to the first image bearing member.
The drawback of this configuration is that, while the test patterns are supplied with the opposite electric charge, the actual printing operation cannot be performed, thereby decreasing productivity. Furthermore, a dedicated charging device is required, thereby increasing the cost.